I can't even guess at how many font styles might exist, but it is a lot. What are the most communally used fonts in ebooks? Does it vary by language or region?
Besides a preference on the broad family of serif fonts over sans-serif ones (see my answer to your other font related question), I don't think that there is any set preference for a particular font type.
Often books from the same publisher tend to have a set of shared fonts (once they have bought licenses for some font types, it is reasonable to broadly use them, both to maximise the investment and to create a unique look and feel for the products of the same company).
Then, one must also consider that there are also many publishers that don't include any font at all in their ebooks; in this situation every ereader uses its standard fonts, unless the users apply their own personal preferences. Even now, serif fonts seem to be the most commonly used (my Kobo defaults to Georgia, and judging from online screenshots, also Kindle and other ereaders default to some kind of serif font).